Good article on the debate of Metal vs. PVC fro DC ducts

[Michael Disorbo]

I have read the debate about using PVC or metal ducts in your home shop. Everyone is concerned about fires and explosions. This article really sheds some good light on the subject.

http://mywebpages.comcast.net/rodec/woodworking/articles/DC_myths.html#summary

Michael

[Jim Becker]

Rod wrote an adaptation of this that appeared in one of the magazines not long ago.

[Chris Barton]

A good article that everyone that is planning a DC system should consider reading. For the cost of a single run of 4" steel ducting 50' long I can duct my entire shop in PVC and buy the blast gates too boot!

[Dale Thompson]

Michael,

I've been in the fire protection engineering field for longer than I will ever admit to! :(

Be that as it may, anyone who has ANY knowledge of electrical theory knows that "grounding" a nonconductor, such as PVC, is a TOTAL oxymoron! It CAN'T be done - PERIOD!!

I ABSOLUTELY agree with 98% of your referenced link. :) The only thing that I take exception to is the rather vague reference to "static" electricity as being the unexplained "possible" cause of explosions in grain/corn/coal dust transfer facilities and silos. In almost 100% of these situations, burning drive motors and/or flaming bearings on motors or conveyors have been proven to be the cause of the explosion. Otherwise, the text, the energy equations and the conclusions are RIGHT ON!! The references to the NFPA standards such as NFPA 68 and 69, specified or not, are valid as are the references to specific portions of the, "NFPA Fire Protection Handbook".

I could go into a LONG dissertation of "Explosion Theory" such as "Prevention", "Oxidant Concentration Reduction", "Combustible Concentration Reduction", "Deflagration Pressure Containment", "Venting", "Enrichment", etc.. - but I WON'T! :)

Suffice it to say; Hook up your home shop DC with your choice of metal, PVC or whatever. Use a minimal amount of "Flex Pipe" to maximize the efficiency of your system, DON'T reduce your pipe size to any machine more than is absolutely necessary and DON'T waste your money on trying to "ground" an "ungroundable" system. :cool: :)

Lastly, the biggest "fire" hazard that I have in MY shop is when I get "hot-headed" over all the mistakes that I make. :( ;) The biggest "explosion" hazard that I have is when my wife is trying to watch "24" and I am running some wood through my router table with a dull bit! :o :eek: :)

Dale T.

[Michael Disorbo]

Dale,

I think I read an older post of yours regarding the same issues. I thought the web site explained a lot. Metal ducting is so expensive and for the amount of dust we are producing, its money better spent on other things like lumber or machinery. I think unless you are running a number of drum sanders at once through a huge system there are other things to worry about. I can show you a nice slot in my father-in-laws shop wall where a 12" jointer knife came out of the cutterhead and traveled 50 feet before hitting that wall. This happened years ago and thank God nobody was in its way. It was a feak accident, but you hear many more of those accidents than you do dust explosions!

Michael

[CPeter James]

Interesting article, now the question is, can I get 5" or 6" PVC and fittings at a reasonable cost?

CPeter

[Cecil Arnold]

Wow Dale, I get all fuzzy when you use those big words ("Oxidant Concentration Reduction", "Combustible Concentration Reduction", "Deflagration Pressure Containment", "Venting", "Enrichment",) but other than that I couldn't agree more. Safety is important, but mostly it's common sense. Too many of us obsess and react to a little information, IMHO.

[Rob Will]

I once had a brain fart and hooked up my mom's clothes dryer with 4" PVC. There was a small section of aluminum pipe hooked directly to the dryer. That in turn attached to PVC. The aluminum pipe stayed clean but static accumulated lint in the PVC to the point that it restricted the air flow!

My WW setup will be a bit heavier than some, for example the planer requires an 8" duct. I can't imagine setting up a heavy duty shop with plastic pipe. I think that LONG radius metal fittings will flow better than most PVC and on top of that, imagine 8" plastic fittings and 8" plastic duct! ($$$)

A good HVAC wholesale house can beat the pants off of the typical WW outlets on metal duct prices. At least we have a good source here. So.... I'm going to stay with spiral metal pipe with gasketed fittings.

Rob

[Rob Blaustein]

I could go into a LONG dissertation of "Explosion Theory" such as "Prevention", "Oxidant Concentration Reduction", "Combustible Concentration Reduction", "Deflagration Pressure Containment", "Venting", "Enrichment", etc.. - but I WON'T! :)

Dale,
How about maybe just a short exegesis on deflagration?
Rob

[Steven Wilson]

Rod's article misses some points though. I'm not concerned with static discharge in PVC causing an explosion, what I read (it's been a few years but a couple of articles from Oregon's OSHA stick in my head as being relevant) that gave me pause was collection of dust on the outside of PVC leading to a secondary fire hazard when the system is upset from a primary ignition source (like a fire starting in your bags). Anyhow, metal is code and if you choose metal piping that is appropriate for your installation (i.e. you don't need the heavy duty elboes from Air Handling Systems if you're running a 3hp DC) the price doesn't have to be vastly more expensive than a similarly configured PVC based system. Large pipe and good flex hose are expensive.

[Norman Hitt]

Interesting article, now the question is, can I get 5" or 6" PVC and fittings at a reasonable cost?

CPeter

6"......Yes, but 5"....NO, they do not make PVC pipe in 5". The solution is to just run 6" all the way to the machines and then either modify the machine outlet to 6", OR reduce to the machine's outlet size right at the machine.

[Bill Lewis]

Rob,
Deflagration has to do the with the rate at which a combustable material is consumed. At a very low rate a particular material may only burn, another material may be capable of combusting at a moderately high rate which is called deflagration, explosives combust at an extremely high rate (think 26000 ft/sec). Black powder deflagrates, but does not explode. It has a burn rate of about 1000 ft/sec. Most would consider the combustion of black powder an explosion, and technically that would be wrong. It does have a lot of force, and can be very damaging, and can have explosive-like results, but that still does not make it an explosive material. This relates directly to Dale's reference to "Deflagration Pressure Containment". Black powder will only procduce explosive-like results when it has been contained, like in a chamber of a gun or some other encasement.

[Cecil Arnold]

Bill, I'm sure you are correct in your definition of explosions, however law enforcement and ATF consider black powder and like devices "low order explosives" and if memory serves they use burn rate as the dividing line. This "burn rate" becomes more understandable when you look at the definition of both fire and explosions which is widely stated as "rapid oxidation (combustion) with the evolution of heat and flames" the difference than hinges on the rate of oxidation.

[Greg Narozniak]

Interesting article, now the question is, can I get 5" or 6" PVC and fittings at a reasonable cost?

CPeter

Check with your local Plumbing Supplier. The line you will want is PVC-SD or "Thin Wall" as it is called. The Next step up from Thin Wall (It really is not needed) would be Schedule 35 (This pipe is the Aqua Green color) Stay away from the Blue and Orange Boxes that sell the Schedule 40 Too Think and Too Expensive.

I think I paid $12 for a 10' run of Thin Wall and a 6x6x6 Wye was $15ish.

Greg

[Dale Thompson]

Dale,
How about maybe just a short exegesis on deflagration?
Rob

OK Rob,
You asked for it but a short comparison is required: :(
- An "explosion" is a rapid release of energy such as from compressed gases. For example, your compressor tank could "explode" if improperly maintained. The hot water heater in your house could "explode" with incredibly devastating results because of what we call a BLEVE (an acronym for Boiling Liquid Expanding Vapor Explosion). :( In neither case would there necessarily be a fire. :eek:

- A "deflagration" is simply an "explosion" involving involving flammable materials such as wood dust or, more commonly, flammable liquid vapors. With modern technology, we can extinguish a "deflagration" because the speed of the flame front is relatively slow - usually around 10 feet/sec. :)

- A "detonation", on the other hand, is a near-instantaneous chemical reaction such as with dynamite or other common "explosives. We cannot suppress these fires because the flame front moves faster than the speed of sound in the materials involved! :(

-With regard to "exegesis", you can most likely find examples of each of the above in the Bible - but that's another story! ;) :) Wanna hear it? :eek:

That's it, Rob! Are you as bored as I am? :confused: I hope this learned ya not to axe me stuff like dis! :)

Dale T.

[Dale Thompson]

Too many of us obsess and react to a little information, IMHO.

Hey Tex,
Ain't it the truth? :) Do I get the "rack", the "noose", the "squad", the "chair", the "chamber" or the "needle"? If given the chance in a poll, I'm sure that all of my "friends" on the Creek would choose, "ALL OF THE ABOVE!". ;) :cool: :D

Dale T.

[Bill Lewis]

... however law enforcement and ATF consider black powder and like devices "low order explosives" and if memory serves they use burn rate as the dividing line... Cecil,
that is actually why I used black powder as an example. And yes, I actully edited out the fact that back powder is considered a low order explosive, or can produce low order explosions. I did not want to confuse some people in my attempt to define "deflagration" as it applies to this topic.
It's interesting on how Dale's explanations/definitions from a "fire persons" point of view differs from an "explosive persons" POV. I work for the Navy, and as such my profession is an engineer and involves the direct use and applications of explosives.

[Dale Thompson]

It's interesting on how Dale's explanations/definitions from a "fire persons" point of view differs from an "explosive persons" POV. I work for the Navy, and as such my profession is an engineer and involves the direct use and applications of explosives.

Bill,
I don't think that we have different points of view at all. :) Every field has its "special" vernacular. The important thing is that co-practitioners in a given field understand the "language". Let's face it, computer geeks, after all these years, have NEVER even learned how to spell bites (bytes). That is probably why the future of computers looks so bleak! :)

Bill, by definition, I have always been stuck withe the points of view of UL (Underwriter's Laboratories), FMRC (Factory Mutual Research corporation) and the NFPA (National Fire Protection Association). The last reference is the MOST recognized. You must admit that the acronym NFPA appears in MANY, or most, GSA Standards, DSA Standards and even MIL Specs. :(

Having been on several NFPA Committees, I find this to be somewhat alarming. NFPA Standards are "concensus" standards. That means, in anyones opinion, that they are COMPROMISES! I'm a true/false guy. As such, I would give NFPA a grade of D+ to C- for both technology and "guts".

As a Navy Engineer, you guys have to get it RIGHT - the FIRST time! On the other hand, what you folks define as an "explosion" would be "technically" defined as a "detonation". However, as far as "technical" definitions are concerned, a rusting car and a "smart bomb" hitting its target are both examples of the oxidation process which gives off both heat and light. It is the RATE of oxidation that makes the difference!! ;) :cool: Isn't it interesting to note that Oxygen is neither flammable nor explosive? :confused:

Lastly, on a personal Navy note, I was a Junior in engineering at the University of Illinois when they were trying to figure out the best way to launch the Polaris missile from a submarine. I saw some of the original tests and they were pretty crude - the rest is history! :cool: :)

I salute the US Navy and its incredible technology!! :D :)

Dale T.